1. Field of the Invention
The present invention relates to an organozirconium composite suitable for use as a raw material for formation of a complex oxide dielectric thin film used in a dielectric memory such as FeRAM (Ferroelectric Random Access Memory) or a dielectric filter using a Metal Organic Chemical Vapor Deposition method (hereinafter referred to as a MOCVD method), relates to a method of synthesizing the same, relates to a raw material solution containing the same, and relates to a method of forming a lead zirconate titanate thin film. More particularly, the present invention relates to an organozirconium composite suitable for formation of a lead zirconate titanate (Pb(Zr,Ti)O3; PZT) thin film and a method of synthesizing the same, and a raw material solution containing the same.
2. Description of the Related Art
DRAM used mainly as a rewritable memory is a volatile memory and causes environmental problems because an electric current must be periodically applied to hold data in memory, and large amounts of power are consumed. Therefore, a ferroelectric memory such as FeRAM, which can store data in a non-volatile memory for a long time and consumes little power consumption and is also interchangeable with DRAM, has attracted special interest. Since the ferroelectric memory has various advantages such as low write voltage, high-speed writing, large number of writing, bit writing and random access, in addition to the above-described features, much research has been performed.
The ferroelectric memory is made of a ferroelectric thin film, as storage capacitors of DRAM, which is provided with a memory function by utilizing a polarization hysteresis phenomenon of the ferroelectric thin film ferroelectric thin film. As the ferroelectric thin film, complex oxide materials having large spontaneous polarization such as PZT are used.
As an organozirconium compound used to form a PZT dielectric thin film, there is known a tetrakis-2,2,6,6-tetramethyl-3,5-heptanedionate zirconium (hereinafter referred to as Zr(thd)4) complex in which a 2,2,6,6-tetramethyl-3,5-heptanedione residue (hereinafter referred to as thd) coordinates to zirconium. As an organolead compound, a bis-2,2,6,6-tetramethyl-3,5-heptanedionate lead (hereinafter referred to as Pb(thd)2) is known. As an organotitanium compound, diisopropoxybis-2,2,6,6-tetramethyl-3,5-heptanedionate titanium (hereinafter referred to as Ti(iPrO)2(thd)2) is known.
It is reported that a film forming temperature of the Zr(thd)4 complex among these complexes shifts from film forming temperatures of other organolead and organotitanium compounds when a PZT dielectric thin film is formed because the decomposition temperature is higher than the decomposition temperatures of the Pb(thd)2 complex and the Ti(iPrO)2(thd)2 complex (see, for example, Anthony C. Jones et al., Journal of the European Ceramic Society, 19 (1999), 1413–1434 (Document 1)).
Therefore, it is also proposed to use a tetratertiary-butoxy zirconium (hereinafter referred to as Zr(tBuO)4) complex having a decomposition temperature lower than that of the Zr(thd)4 complex as a raw material of a PZT thin film. However, it is very difficult to handle the compound disclosed in the above-mentioned document because it is highly reactive with air.
As a novel organozirconium compound for MOCVD to solve the above-described problems, a diisopropoxybis-2,2,6,6-tetramethyl-3,5-heptanedionate zirconium (hereinafter referred to as Zr(iPrO)2(thd)2) complex, a ditertiary-butoxybis-2,2,6,6-tetramethyl-3,5-heptanedionate zirconium (hereinafter referred to as Zr(tBuO)2(thd)2) complex and a Zr2(iPrO)6(thd)2 complex are disclosed (see, for example, PCT International Publication Number WO98/51837 (Document 2)). These novel organozirconium compounds are superior to the above-described conventional organozirconium compounds because a film can be formed over a broad temperature range.
As another novel organozirconium compound for MOCVD, an isopropoxytris 2,2,6,6-tetramethyl-3,5-hepntanediolnate zirconium (hereinafter referred to as Zr(iPrO)(thd)3) complex is proposed (see, for example, OKUHARA et al., Preliminary Manuscript of 47th Annual Meeting of The Japan Society of Applied Physics (May, 2000), p540 (Document 3)). The proposed organozirconium compound is a monomer and has properties such as high vapor pressure and high solubility in solvent.
However, the organozirconium compound disclosed in Document 2 had the following drawbacks. That is, when mixed with an organolead compound to form a PZT dielectric thin film by the MOCVD method, the organozirconium compound is liable to react with the organolead compound and is not sufficiently vaporized when vaporized and introduced into a film forming chamber, and thus almost all of the compound is left as a residue.
The organozirconium compound disclosed in Document 3 has the following problems. That is, the compound per se is liable to leave a large amount of a vaporization residue and, furthermore, the compound is not sufficient vaporized upon introduction into a film forming chamber after vaporization, and therefore a large amount of the compound is left as a residue.
As a zirconium material for solving the above-described problems, the present inventors tentatively used a Zr(tBuO)(thd)3 complex and a Zr(tAmO)(thd)3 complex. However, these zirconium materials cause a disproportionate reaction to form Zr(thd)4 over time in the case of dissolving in an organic solvent to obtain a raw material solution.
Also a study on a compound having a low decomposition temperature has been made and the subject of the study is tetrakis-2,6-dimethyl-3,5-heptanedionate zirconium (hereinafter referred to as Zr(dhd)4) in which a 2,6-dimethyl-3,5-heptanedione residue (hereinafter referred to as dhd) coordinates to zirconium. However, Zr(dhd)4 has a low decomposition temperature compared to an organotitanium compound such as Ti(iPrO)2(thd)2 and has a problem in that it is difficult to control the film composition.